Around 2265, she was promoted to captain, and given command of the USS Bombay. In that year, they were assigned to Starbase 47 (Vanguard) as a support vessel as part of Operation Vanguard under Commodore Reyes's command. The Bombay was the workhorse of the operation, patrolling the region, carrying supplies to colonies and outposts and surveying star systems, but Captain Gannon carried out the increasingly overwhelming assignments without complaint.

On a supply mission to the Ravanar IV Starfleet outpost, the Bombay was ambushed by six Tholiancruisers, intent on wiping out the outpost on the planet. Against the terrible odds, Captain Gannon decided to try and save the outpost, eventually destroying three Tholian vessels. When the Bombay was all but crippled, most of its bridge crew were dead and Gannon herself badly wounded, she used a tractor beam to pull a Tholian cruiser close to her ship. As her final act, before succumbing to ther injuries, Gannon activated Bombay' self-destruct system with HelmsmanOriana D'Amato's assistance, destroying her ship and another enemy vessel. (VANnovel: Harbinger)